function [theta, vec] = convertUnitQuaternion2AngleAxis (e)

    e0 = e(1);
    e1 = e(2);
    e2 = e(3);
    e3 = e(4);

    theta = 2*acos(e0);
    x = e1/sin(0.5*theta);
    y = e2/sin(0.5*theta);
    z = e3/sin(0.5*theta);
    
    vec = [x y z]';